Chapter 18. mod_perl Data-Sharing Techniques

Contents:

In this chapter, we discuss the ways mod_perl makes it possible to
share data between processes or even between different handlers.

18.1. Sharing the Read-Only Data in and Between Processes

If you need to access some data in your code that's
static and will not be modified, you can save time and resources by
processing the data once and caching it for later reuse. Since under
mod_perl processes persist and don't get killed
after each request, you can store the data in global variables and
reuse it.

For example, let's assume that you have a rather
expensive function, get_data( ), which returns
read-only data as a hash. In your code, you can do the following:

This code creates a global hash, %CACHE, which is
undefined when the code is executed for the first time. Therefore,
the get_data( ) method is called, which hopefully
populates %CACHE with some data. Now you can
access this data as usual.

When the code is executed for the second time within the same
process, the get_data( ) method will not be called
again, since %CACHE has the data already (assuming
that get_data( ) returned data when it was called
for the first time).

Now you can access the data without any extra retrieval overhead.

If, for example, get_data( ) returns a reference
to a list, the code will look like this:

Here we use the enum pragma to create constants
that we will use in accessing the array reference. In our example,
FIRST equals 0, SECOND equals
1, etc. We have used the RA_ prefix to indicate
that this variable includes a reference to an array. So just like
with the hash from the previous example, we retrieve the data once
per process, cache it, and then access it in all subsequent code
re-executions (e.g., HTTP requests) without calling the heavy
get_data( ) method.

This is all fine, but what if the retrieved data set is very big and
duplicating it in all child processes would require a huge chunk of
memory to be allocated? Since we assumed that the data is read-only,
can we try to load it into memory only once and share it among child
processes? There is a feasible solution: we can run the
get_data( ) method during server startup and place
the retrieved data into a global variable of some new package that we
have created on the fly. For example, let's create a
package called Book::Cache, as shown in Example 18-1.

Example 18-1. Book/Cache.pm

Now when the child processes get spawned, this data is available for
them all via a simple inclusion of the module in the
handler's code:

use Book::Cache ( );
...
$foo = $Book::Cache::DATA{bar};
...

Be careful, though, when accessing this data. The data structure will
be shared only if none of the child processes attempts to modify it.
The moment a child process modifies this data, the copy-on-write
event happens and the child copies the whole data structure into its
namespace, and this data structure is not shared anymore.